Optical frequency combs (OFCs) are useful tools for many applications including optical clocks, precision frequency/time transfer, low phase noise microwave generation, astronomical spectrograph calibration, molecular spectroscopy, coherent LIDAR, and arbitrary optical/RF waveform generation. The main advantage of OFCs arises from the fact that thousands of highly coherent optical frequencies are accurately and precisely defined with only two degrees of freedom, namely, the carrier-envelop-offset frequency (CEO frequency) and the repetition rate of the femtosecond laser pulse train. Despite years of research and development effort from both academia and industry, OFCs are generally currently available only at leading metrology labs that can build the laser system themselves or have enough resources to purchase expensive commercial OFCs. OFCs may find more widespread use in practical applications if they are less expensive, easier to build, and more robust such that they can work outside of a controlled laboratory environment. This is particularly true for applications needing more than one OFC such as for dual-comb spectroscopy (DCS).
In DCS, two broadband mutually coherent OFCs (at least during the time of measurement) working at the same center frequency but having a slight difference in repetition rates are needed. DCS can achieve high spectral resolution and short acquisition time simultaneously since no moving part is involved. In addition, single-shot, high signal-to-noise ratios over a large spectrum bandwidth have been demonstrated with the use of tightly phase-locked coherent OFCs. One of the ongoing research efforts is focusing on simplifying the experimental setup for a high quality DCS. A notable result in this direction is the adaptive sampling technique reported in T. Ideguchi et al., “Adaptive real-time dual-comb spectroscopy,” Nat. Commun. 5, 3375 (2014). It turned out that the demanding phase-locking requirement of the two OFCs can be removed by an adaptive sampling technique using specially designed electronics. Recently, another technique has been demonstrated that allowed DCS of an acetylene gas cell using two free-running mode-locked fiber lasers. The experimental setup is simpler but it did not achieve single-shot measurement due to the poor signal-to-noise. DCS has also been reported with the use of a single mode-locked laser and a Dazzler, but the spectral resolution and noise performance were limited.
DCS is a form of Fourier transform spectroscopy. In order to simplify its requirements, the traditional moving mirror is replaced by sampling one OFC with a second OFC which operates at a slightly different repetition rate. The two combs are typically generated from two different laser cavities so they are not phase coherent. DCS requires the two combs to be phase coherent during the time of measurement since narrow absorption lines would be washed out otherwise. One way to implement DCS in a phase coherent manner is to phase lock the two combs using electronics. Each comb has two degrees of freedom, so four servo locks are needed on top of the knowledge of the CEO frequency for each comb (which typically requires octave spanning supercontinuum generation for f-to-2f detection). This approach is not simple but it works and exhibits the best DCS performance achieved so far.